The present invention relates to refractory coatings on articles, such as bearings, bushings, couplings or rollers, used in a molten metal environment, such as in aluminizing or galvanizing processes.
Aluminizing and galvanizing processes take place at very high temperatures. For example, a 100% aluminum aluminizing bath typically can have a temperature of 1,325xc2x0 F. (720xc2x0 C.). In order to run these processes effectively, in an industrial context, it is necessary to have various metal parts, such as bearings, bushings, couplings or rollers, immersed wholly or partly in the aluminizing or galvanizing bath such that the metal pieces or strip to be coated can effectively and efficiently move through the bath. Such baths tend to be very destructive of these metal parts due to rapid metallurgical alloying of the reactive molten metal with the commonly used structural alloys. This results in frequent stoppage of the process so that the parts can be replaced to allow the process to continue. The more frequently the process has to be stopped to replace such parts, the less efficient it is and the greater the running cost of the process. Therefore, designing metal parts which can remain intact in the aluminizing or galvanizing baths for a longer period of time would be highly desirable. It is that goal which the present invention addresses.
The use of refractory metals as coatings for metal parts is known in the art. For example, U.S. Pat. No. 2,839,292, Bellamy, issued Jun. 17, 1958, describes that molybdenum and tungsten coatings on metal parts are highly resistant to attack when immersed in molten aluminum. The coatings can be applied by spraying (which is preferred), plating or cladding. There is no disclosure of the use of high density refractory metal coatings. The patent utilizes an outer ceramic layer to protect the parts from oxidation.
U.S. Pat. No. 5,360,657, Wood, et al., issued Nov. 1, 1994, describes a molybdenum (3% to 9% Mo) boron alloy which is said to show excellent resistance to attack by molten zinc. The patent deals very specifically with molten zinc, discussing its low viscosity and surface tension, and does not deal with molten aluminum at all. Further, there is no discussion of tungsten coatings or of application by any means other than thermal spray. Finally, the patent (at column 5, lines 20-27) demonstrates that a pure molybdenum coating is clearly inferior to the molybdenum-boron alloy coating.
U.S. Pat. No. 5,759,142, Perdikaris, issued Jun. 2, 1998, discloses an aluminizing process guide roll, said to provide longer wear, which utilizes a multi-layer coating on the roll""s surface including a first coating layer comprising MCrAlY metal in which M is Ni or Co, and a second coating layer of a refractory oxide of aluminum, zirconium, silicon or chromium. The patent teaches that the MCrAlY coating must be placed directly against the surface of the guide roll and that there not be any intermediate layers between them.
U.S. Pat. No. 5,310,476, Sekhar, et al., issued May 10, 1994, teaches the formation, utilizing a micropyretic heating process, of a coating made from a refractory material, such as molybdenum or niobium compounds. These coatings are said to be inert to damage from molten aluminum.
U.S. Pat. No. 5,370,372, Eckert, issued Dec. 6, 1994, describes a ladle used for working with molten metal (such as aluminum) which is said to be resistant to damage caused by the molten metal. The ladle may be made from niobium or molybdenum, among other materials, and has a refractory oxide coating which comprises at least one of alumina, zirconia, yttria-stabilized zirconia, magnesia, magnesium titanate, mullite, or a combination of alumina and titania.
The present invention relates to two different, but related, refractory metal coatings used to protect metal parts placed in a molten metal environment.
The first embodiment is an article used in contact with molten aluminum or molten zinc, particularly molten aluminum, which is coated with a high density coating consisting of a metal selected from the group consisting of Group Vb, VIb and VIIb metals (in pure or alloyed form). The coatings generally have a thickness of from about 0.01 to about 0.30 inch. The articles are generally in the form of a bearing, bushing, sleeve, coupling or roller. Preferred metals are the Group VIb) metals, particularly molybdenum and tungsten, most particularly in pure form. Examples of high density coating application methods which may be used in this embodiment of the present invention include plasma-transferred arc, welding overlay, and high-velocity arc spraying processes. These parts may optionally be provided with an aluminum-based or similarly effective overlay to provide oxidation protection to the part prior to immersion.
The second embodiment of the present invention is a roll for guiding strip steel through a high temperature aluminizing bath, said roll comprising a roll body having a surface in guiding contact with said steel strip within said bath and a multi-layer coating on said roll""s surface for contact with said steel strip, said multi-layer coating including a first primer layer on said roll""s surface, comprising a group Vb, VIb or VIIb metal, particularly tungsten or molybdenum, either as pure metal or an alloy; a second layer comprising MCrAlY metal in which M is Ni or Co, said MCrAlY metal being thermally sprayed on said primer layer; and a third layer comprising a refractory metal oxide of aluminum, zirconium, silicon or chromium, said third layer having a higher porosity than said second layer to better accommodate thermal expansion coincident with bath immersion without disruption of the third layer surface, while the less porous MCrAlY metal second layer and the first layer maintain effective metallurgical corrosion protection of said roll surface.
All percentages and ratios given herein are xe2x80x9cby weight,xe2x80x9d unless otherwise specified.
In its first aspect, the present invention encompasses articles used in contact with molten aluminum or molten zinc which are coated with a high density coating consisting of certain specific refractory metals. The articles are any of a variety of articles which typically would be immersed either wholly or partially in a bath of molten aluminum or molten zinc during the aluminizing or galvanizing process and can include, for example, bearings, bushings, sleeves, couplings and rollers. The metals which are utilized for the coating are selected from the group consisting of Group Vb, VIb and VIIb metals, including, for example, molybdenum, tungsten, niobium, tantalum, and rhenium. The Group VIb metals are preferred, with molybdenum and tungsten being particularly preferred.
The metals may be applied in either pure or alloyed form, with pure form being preferred. The metals are applied to the article in the form of a high density coating having controlled and minimized microstructural porosity. As used herein, xe2x80x9chigh densityxe2x80x9d means a coating which has a porosity of no greater than about 15%, preferably no greater than about 5%, based on the metallographic examination technique common in metallurgical practice. Such application methods include, for example, plasma transferred arc (which is preferred), welding overlay, and high velocity plasma arc spraying in either air, protective cover (gas or water) or low atmospheric pressure chamber. These application methods provide a coating which is dense and which forms an intimate bond with the substrate. The coating could also be formed as a sleeve which is applied and bonded to the substrate. In the plasma transferred arc method, the coating material (e.g., molybdenum) is melted to form the coating, but the substrate is not extensively melted; the coating therefore is not diluted significantly upon application. Lower density application methods, such as thermal spray, plating or cladding are not suitable for use in the present invention. When applied as described herein, the articles of the present invention exhibit a much longer useful life when utilized in a molten aluminum or molten zinc bath, most preferably a molten aluminum bath. The coatings generally have a thickness of from about 0.01 to about 0.30 inch, more preferably from about 0.02 to about 0.06 inch. In coatings useful in the present invention, thickness and density are inversely related, i.e., thinner coatings can be used if they have higher densities. These coatings are both wear resistant and protect the underlying article from wear by the molten metal.
The article of the present invention may additionally include, over said high density refractory metal coating, an outer coating of an oxidation-resistant material. Such materials are well known in the art and, for example, include aluminum metal, zinc metal, NiCr, MCrAlY in which M is Ni or Co, and mixtures thereof. It is preferred that this outer layer be made from aluminum and have a thickness of from about 0.001 to about 0.015 inch. This outer coating may include application of a metal or ceramic pigmented paint, as well as metal spray techniques, suitable for oxidation protection during pre-heat exposure.
The present invention also encompasses an apparatus for coating metal strip with aluminum or zinc (preferably aluminum) comprising a reservoir for holding molten aluminum or molten zinc and, positioned such that it is at least partially submersed in said molten aluminum or zinc, an article selected from bearings, bushings, couplings and rollers, said article being coated with a high-density coating consisting of a metal selected from the group consisting of Group Vb, VIb and VIIb metals, such as molybdenum, tungsten, niobium, tantalum and rhenium, in pure or alloyed form. These coatings have a thickness of from about 0.01 to about 0.30 inch, preferably from about 0.02 to about 0.06 inch.
In its second embodiment, the present invention encompasses rollers used for guiding steel strip through a high temperature aluminizing bath. While such rollers are known in the art (see, for example, U.S. Pat. No. 5,759,142, Perdikaris, issued Jun. 2, 1998, incorporated herein by reference), the rollers of the present invention provide significant and unexpected benefits in terms of extended useful life when used in a molten aluminum bath. Specifically, and surprisingly, while the Perdikaris patent describes aluminizing rollers which include an MCrAlY layer with an outer coating of refractory oxide, that patent specifically teaches that, in order to be effective, the MCrAlY layer must be situated directly against the surface of the aluminizing roller. The present invention is not only based on the fact that this is not correct, i.e., that the MCrAlY layer does not have to be directly against the outer surface of the roller, but also on the fact that by incorporating a first (primer) layer made from a refractory metal, preferably tungsten or molybdenum, either as pure metal or as an alloy, the durability or useful life of the rollers is significantly improved.
The basic aluminizing rolls utilized in the present invention are those utilized for guiding steel strip through a high temperature aluminizing bath. These rolls are well known in the art and comprise a roll body which has a surface in guiding contact with the steel strip within the bath. Such rolls are disclosed, for example, in U.S. Pat. No. 5,759,142, Perdikaris, issued Jun. 2, 1998, incorporated herein by reference. On the surface of this roll of the present invention is placed a first (primer) layer of a Group Vb, VIb or VIIb metal, preferably tungsten or molybdenum, either as pure metal or in alloy form. It is preferred that the coating be applied as a pure metal. The coating may be applied by any method known in the art (for example, thermal spray, plating, cladding), with thermal spray application being preferred. The high-density coatings, described above (e.g., plasma transferred arc) may also be utilized. This initial layer generally has a thickness of from about 0.002 to about 0.05 inch, with a thickness of from about 0.002 to about 0.007 inch being preferred.
The second layer, which is placed on top of and adjacent to the first (primer) layer, comprises MCrAlY metal in which M is nickel (Ni) or cobalt (Co). These coatings are well-known in the art and are described in, for example, U.S. Pat. No. 5,759,142, Perdikaris, Jun. 2, 1998, incorporated herein by reference. Such layer may be applied in any manner known in the art, with thermal spray application being preferred. The preferred material for use in forming the second layer is NiCrAlY. The coating is generally applied to a thickness of from about 0.002 to about 0.015 inch, with a thickness of from about 0.004 to about 0.006 inch being preferred. It is preferred that the second layer has a porosity in the range of from about 1% to about 10%.
The third layer, which is placed on top of and adjacent to the second layer, comprises a refractory metal oxide of aluminum (Al), zirconium (Zr), silicon (Si) or chromium (Cr), or mixtures thereof. This third layer has a higher porosity than the second layer in order to better accommodate thermal expansion of the metal member coincident with pre-heat and subsequent immersion into the high temperature molten aluminum bath (around 1325xc2x0 F.). The third layer may be applied in any manner conventionally known in the art, although thermal spraying is preferred. The preferred material for use in this third layer is a refractory oxide mixture of zirconia and alumina. The third layer is generally applied to a thickness of from about 0.01 to about 0.05 inch (although it can be thicker), with from about 0.012 to about 0.032 inch being preferred, the third layer preferably having a porosity of from about 4% to about 15%. Once again, such layers are well-known in the art and are disclosed, for example, in U.S. Pat. No. 5,759,142, Perdikaris, issued Jun. 2, 1998, incorporated herein by reference.
Optionally, between the second and third layers, the roll may incorporate a blend layer which includes a mixture of from about 25% to about 75% (by weight) of the material from the second layer and from about 75% to about 25% (by weight) of the material from the third layer. This blend layer helps to accommodate thermal expansion of the roll. Primer may also be included in this blend layer.
Finally, an optional sealer coating (such as boron nitride) may be placed on the surface of the coated roll to provide an initial barrier.
In use, when the otherwise inert outer oxide layer, which is protecting the roll from attack by molten aluminum, is physically breached by the liquid metal, through inevitable stress/strain cracks, the refractory metal components which may be incorporated into the underlying second, as well as being first (primer), layer are far more resistant to the encroaching aluminum than the MCrAlY coating. Using these elements, or alloys of them, as the metallic component of the coating will make the composite coating less vulnerable to deterioration by aluminiding (i.e., metallurgical reaction with the aluminum). Secondly, the coating will hold off aluminiding and metallurgical corrosion of the roll itself from aluminum diffusion. The benefit of this concept is extended life for the coated roll, as the bulk applied coating structure, having preferred porosity, with composite layering including the aluminide-deferring refractory metal components, remains protective of the roll for longer runs.